1. Field of the Invention
The present invention relates to an environment-control-oriented scan type electron microscope.
2. Related Background Art
In a typical scan type electron microscope, secondary electrons emitted from a sample (target) disposed in a vacuum are detected by luminescence of a scintillator. In contrast with this, according to an environment-control-oriented scan type electron microscope, the secondary electrons from the sample disposed in a low-pressure gas such as a water vapor are gas-amplified by the gas. The thus amplified secondary electrons are detected directly by a detection electrode. This environment-control-oriented scan type electron microscope is capable of viewing a variety of samples that could not be viewed by ordinary electron microscopes.
In an ordinary low-acceleration scan type electron microscope, however, a primary electron beam incident on the sample is substantially equivalent to a quantity of the secondary electrons emitted from the sample. Contrastingly, in the environment-control-oriented scan type electron microscope, an acceleration voltage of the electron beam is relatively large. Hence, the quantity of the secondary electrons emitted from the sample is smaller than a quantity of the primary electron beam incident on the sample. For this reason, a negative charge-up (electrification) of the sample is easy to take place in the environment-control-oriented scan type electron microscope. A mechanism for neutralizing the negative charge of the sample is therefore required.
FIG. 9 shows one example of the conventional environment-control-oriented scan type electron microscope. Referring to FIG. 9, an electron gun (unillustrated) is installed in a vacuum chamber 1 (consisting of, as a matter of fact, a plurality of subchambers partitioned by an aperture for differential exhausting). The vacuum chamber 1 is adjacent to a sample chamber 2 (its outer frame is not shown) for housing a sample 7 through a pressure limiting aperture plate 3. An objective lens composed of an electromagnetic lens is provided along the circumference of the pressure limiting aperture plate 3 through an insulator 4. The sample chamber 2 is supplied with a gas (e.g., a water vapor) for effecting a gas amplification from an unillustrated supply source. At the same time, a gas pressure in the sample chamber 2 is kept on the order of 0.1 Torr to several tens of Torr by means of an unillustrated vacuum pump. Further, the gas in the sample chamber 2 flows via the pressure limiting aperture plate 3 into the vacuum chamber 1. However, a gas pressure in the vacuum chamber 1 is kept under a smaller pressure (state of a higher degree of vacuum) on the order of 10.sup.-2 to 10.sup.-3 Torr just above, e.g., the pressure limiting aperture plate 3 than in the interior of the sample chamber 2 by means of a vacuum pump 6. In fact, as explained above, the vacuum chamber 1 is partitioned by the aperture into the plurality of subchambers. The vacuum pump is provided in each subchambers. The electron gun is installed in the subchamber exhibiting the highest degree of vacuum.
A sample 7 defined as a view target is housed in the interior of the sample chamber 2. Further, in this conventional example, the pressure limiting aperture plate 3 serves as a secondary electron detector. A positive voltage with respect to the sample 7 is applied to the pressure limiting aperture plate 3 from a variable voltage source 8. A secondary electron signal obtained from the pressure limiting aperture plate 3 is led via a preamplifier 9 to an unillustrated processor.
When viewing the sample 7, the sample 7 is irradiated with a primary electron beam emitted from the electron gun in the vacuum chamber 1 via an aperture 3a of the pressure limiting aperture plate 3. At this time, the secondary electrons emitted from the sample 7 are gas-amplified by the gas existing in the sample chamber 2 while migrating toward the pressure limiting aperture plate 3 because of an electric field of the pressure limiting aperture plate 3 serving as the secondary electron detector. Positive ions produced as a result of this fall on the sample 7, thereby neutralizing a negative charge of the sample 7 which is generated with the irradiation of the electron beam. Further, the gas-amplified secondary electrons are taken in the pressure limiting aperture plate 3 serving as the secondary electron detector. A secondary electron signal thereof is taken in an external processor via a preamplifier 9.
FIG. 10 illustrates another example of the conventional environment-control-oriented scan type electron microscope. In this conventional example, the pressure limiting aperture plate and the secondary electron detector are separately provided. Referring to FIG. 10, the portions corresponding to those in FIG. 9 are marked with the like symbols, and the detailed explanations thereof will be omitted.
Referring again to FIG. 10, a ring-like secondary electron detector 10 having a minor diameter of several mm is disposed along the circumference of the pressure limiting aperture plate 3 upwardly of the sample 7. A positive voltage is applied to the secondary electron detector 10 from a variable voltage source 11. The secondary electron signal from the secondary electron detector 10 is also taken in the unillustrated processor via a preamplifier 12. Further, the positive voltage with respect to the sample 7 is also applied to the pressure limiting aperture plate 3 from the variable voltage source 8.
In the apparatuses shown in FIGS. 9 and 10, however, a much larger amount of excessive positive ions than an amount of incidence of the primary electron beam fall on the sample 7. This results in a positive charge-up of the sample 7. As a consequence of this charge-up, a potential difference between the secondary electron detector and the sample is decreased; and a gas amplification rate of the secondary electrons is reduced. For this reason, there exist inconveniences in which a gain of the secondary electron detector declines, and an SN ratio of a view image obtained decreases.
A reduction in the potential difference between the secondary electron detector and the sample due to the charge-up of the sample 7 may be compensated by stepping up a voltage of the secondary electron detector. However, in such case, a potential difference between the secondary electron detector and a metal (e.g., lens) at an earth potential in the vicinity of this detector in turn becomes too large. A trace amount of discharge is caused therebetween. This results in noise on the view image, so that is becomes impossible to improve the SN ratio.